JP2012038625A - Conductive fine particle-dispersed paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive fine particle-dispersed paste capable of being suitably used for forming a back electrode of a solar cell and excellent in storage stability.SOLUTION: A conductive fine particle-dispersed paste used for forming a back electrode of a solar cell contains: at least one selected from a group consisting of ethyl cellulose, a (meth)acrylic resin and a polyvinyl acetal resin; an organic solvent; an organic compound solid at ordinary temperature; and conductive fine particles. The organic compound solid at ordinary temperature is an organic compound that is solid at ordinary temperature, has two or more of hydroxyl groups, and has a ratio of (carbon number/hydroxyl group number) of less than 5.

Description

The present invention relates to a conductive fine particle-dispersed paste that can be suitably used for forming a back electrode of a solar cell because it has excellent storage stability and a low electrical resistance value.

In recent years, photovoltaic power generation has attracted attention as a clean energy alternative to fossil fuel, and the development of solar cells is progressing. As a solar cell, a device in which an electrode is formed on a silicon semiconductor substrate is known. FIG. 1 is a cross-sectional view schematically showing the structure of a general solar cell.

As shown in FIG. 1, the solar cell element has an n-type impurity layer 2 having a thickness of 0.3 to 0.6 μm formed on the light receiving surface side of a p-type silicon semiconductor substrate 1 having a thickness of 200 to 300 μm. Furthermore, an antireflection film 3 and a grid electrode 4 are formed on the n-type impurity layer 2. A back electrode layer 5 is formed on the back side of the p-type silicon semiconductor substrate 1.

The back electrode layer 5 is usually formed by applying a paste containing aluminum powder, glass frit, and organic vehicle by screen printing or the like, drying, and firing at a temperature of 660 ° C. (melting point of aluminum) or higher. . In this firing step, aluminum diffuses into the p-type silicon semiconductor substrate 1, thereby forming an Al—Si alloy layer 6 between the back electrode layer 5 and the p-type silicon semiconductor substrate 1. A p ⁺ layer 7 is formed as an impurity layer by atomic diffusion. The presence of the p ⁺ layer 7 can provide a BSF (Back Surface Field) effect that prevents recombination of electrons and improves the collection efficiency of generated carriers.

In recent years, the demand for cost reduction of solar cells has increased, and as a method for this, it has been studied to reduce the thickness of a silicon semiconductor substrate. However, since the thermal expansion coefficient of silicon and aluminum differ greatly, if the silicon semiconductor substrate becomes thinner, internal stress occurs due to the difference in thermal expansion coefficient, and the back electrode layer is formed after the paste is baked. There is a problem that the silicon semiconductor substrate is deformed so that the side is concave, and warping or cracking occurs.

In order to solve such a problem, in Patent Document 1, the aluminum-containing paste is thinly applied to the entire back surface of the semiconductor substrate, and the aluminum-containing paste is again applied to the portion to be thickened from above, and then fired. A method of forming a back electrode layer composed of two or more layers on the back surface of a semiconductor substrate is disclosed. Patent Document 2 discloses a method of forming backside electrodes in a grid pattern on the backside of a semiconductor substrate.

Patent Document 3 discloses a method using an aluminum powder, a glass frit, an organic vehicle, and an aluminum-containing organic compound, and Patent Document 4 has a smaller coefficient of thermal expansion than aluminum and a melting temperature, A method of using a paste composition containing an inorganic compound, an aluminum powder, and an organic vehicle in which either the softening temperature or the decomposition temperature is higher than the melting point of aluminum is disclosed. Furthermore, in Patent Document 5, warping of the silicon semiconductor substrate is reduced by adding at least one of organic compound particles and carbon particles to the conventional paste composition to suppress shrinkage of the aluminum electrode during firing. A method is disclosed.

The conventional paste compositions shown in the above documents tend to increase the composition ratio of the aluminum fine particles in the paste in order to reduce the resistance value of the aluminum electrode or reduce the warpage of the substrate after firing. is there. In particular, in a composition in which the aluminum fine particles in the paste are 70% or more, it is difficult to maintain the dispersion state with a small amount of binder resin, and there is a problem that the aluminum powder settles in about 1 to 2 weeks after preparation. It was. In order to ensure storage stability, it is conceivable to increase the content of the binder resin in the paste and increase the viscosity. However, by increasing the resin, more organic substances remain in the short-time baking process. There is also a problem that the electric resistance value becomes high.

JP 2002-217435 A JP 2002-141533 A JP 2000-90734 A Japanese Patent Laid-Open No. 2003-223813 JP 2004-134775 A

An object of the present invention is to provide a conductive fine particle-dispersed paste that can be suitably used for forming a back electrode of a solar cell because it has excellent storage stability and a low electrical resistance value.

The present invention relates to a conductive fine particle-dispersed paste used for forming a back electrode of a solar cell, at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin and polyvinyl acetal resin, and an organic solvent A normal temperature solid organic compound and conductive fine particles, and the normal temperature solid organic compound is solid at normal temperature, has two or more hydroxyl groups, and (carbon number / hydroxyl number) is 5 It is a conductive fine particle-dispersed paste which is less than an organic compound.
The present invention is described in detail below.

As a result of intensive studies, the present inventors are solid at room temperature in addition to at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin and polyvinyl acetal resin, an organic solvent and conductive fine particles. The conductive fine particle dispersion paste having two or more hydroxyl groups and containing an organic compound having (carbon number / hydroxyl number) of less than 5 is surprisingly compared with the case where no organic compound is contained, It has been found that the storage stability of the paste is improved. Moreover, since it can reduce resin content compared with the case where an organic compound is not contained, it also discovered that an electrical resistance value could be reduced and came to complete this invention.

In the present invention, the reason why the storage stability of the paste is improved is not clear, but in addition to the effects of the hydrophilic functional group in the ethyl cellulose, the (meth) acrylic resin and the polyvinyl acetal resin, and the hydroxyl group of the organic solvent. This is probably because the conductive fine particles can be dispersed and stabilized by the effect of the hydroxyl group of the organic compound that is solid at room temperature. Further, the reason why the content of the resin can be suppressed to a low level is not clear, but the hydrophilic functional group in the ethyl cellulose, the (meth) acrylic resin and the polyvinyl acetal resin, the hydroxyl group of the organic solvent, and the room temperature solid This is because the hydroxyl group of the organic compound strongly interacts with hydrogen bonds or the like, and as a result, the electrical resistance value can be lowered.

The conductive fine particle dispersed paste of the present invention contains at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin, and polyvinyl acetal resin.
Although the said ethyl cellulose is not specifically limited, When performing screen printing, grades of ethyl cellulose, such as STD4, STD10, STD45, STD100, are preferable.

The (meth) acrylic resin is not particularly limited as long as it decomposes at a low temperature of about 350 to 400 ° C., but methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl ( (Meth) acrylate, tert-butyl (meth) acrylate, isobutyl (meth) acrylate, cyclohexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, isobornyl (meth) acrylate, n-stearyl (meth) acrylate, benzyl (meth) A polymer comprising at least one selected from the group consisting of acrylate and (meth) acrylic monomers having a polyoxyalkylene structure is preferred. Here, for example, (meth) acrylate means acrylate or methacrylate.
Among them, polymethyl methacrylate (polymer of methacrylate, Tg 105 ° C.) having a high glass transition temperature (Tg) is preferable because a high viscosity can be obtained with a small amount of resin. Polymethacrylate is also excellent in low temperature degreasing properties. Furthermore, since the conductive fine particle-dispersed paste of the present invention contains an organic compound described later, a polymer having a component derived from butyl methacrylate, isobutyl methacrylate, or cyclohexyl methacrylate is preferable.

Furthermore, the conductive fine particles preferably have a hydrophilic functional group at the molecular side chain or at the molecular end because of high interaction with nitrogen functional groups such as amino groups, hydroxyl groups and the like.
When a (meth) acrylic resin having a hydrophilic functional group, such as a nitrogen functional group such as an amino group, a hydroxyl group, or the like is used at the molecular side chain or molecular end, one of the (meth) acrylic resins The molecular terminal is adsorbed on the surface of the conductive fine particles, and the other is extended to the organic solvent side, preventing re-aggregation of the conductive fine particles and improving the dispersion stability, so that the storage stability is improved.

The method for producing the (meth) acrylic resin having a hydrophilic functional group in the molecular side chain is not particularly limited, and examples thereof include a method of copolymerizing a (meth) acrylic acid alkyl ester monomer having a hydrophilic functional group. It is done.

Examples of the (meth) acrylic acid alkyl ester monomer having a hydrophilic functional group include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 4 -Hydroxybutyl (meth) acrylate, cyclohexanedimethanol mono (meth) acrylate, neopentyl glycol mono (meth) acrylate, glycerol mono (meth) acrylate, trimethylolpropane mono (meth) acrylate, pentaerythritol mono (meth) acrylate, (Meth) acrylamide, (meth) acrylonitrile, N-vinylpyrrolidone, N- (meth) acryloylmorpholine, 2-N, N-dimethylaminoethyl (meth) acrylate, 2-N, N-diethyl Aminoethyl (meth) acrylate. Among the above-mentioned (meth) acrylic acid alkyl ester monomers having each functional group, a residue of decomposition residue is particularly small after degreasing, and therefore a (meth) acrylic acid alkyl ester monomer having a hydroxyl group is suitable. is there.

When it has a segment derived from the (meth) acrylic acid alkyl ester monomer having the hydrophilic functional group, the content of the segment derived from the (meth) acrylic acid alkyl ester monomer having the hydrophilic functional group is It is preferable that it is 30 weight% or less. If the content of the segment exceeds 30% by weight, thermal decomposition at low temperatures may be impaired, soot attached to the conductive fine particles may increase, and residual carbon in the sintered body may increase. More preferably, it is 20% by weight or less.

The production method of the (meth) acrylic resin having a hydrophilic functional group at the molecular end is not particularly limited. For example, a (meth) acrylic monomer is subjected to a free radical polymerization method in the presence of a chain transfer agent having a hydrophilic functional group. In the presence of a polymerization initiator having a hydrophilic functional group, a method of polymerizing or copolymerizing by a conventionally known method such as a living radical polymerization method, an iniferter polymerization method, an anionic polymerization method, or a living anion polymerization method, ) A method of polymerizing or copolymerizing an acrylic monomer by a conventionally known method such as a free radical polymerization method, a living radical polymerization method, an iniferter polymerization method, an anion polymerization method, or a living anion polymerization method. These methods may be used independently and may use 2 or more types together.

Moreover, you may manufacture the (meth) acrylic resin which has a hydrophilic functional group in the said molecular terminal using the azo polymerization initiator which has a hydrophilic functional group.
The azo polymerization initiator is not particularly limited, and 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis [2- (2-imidazoline). -2-yl) propane] disulfate dihydrate, 2,2′-azobis (2-methylpropionamidine) dihydrochloride, 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropion Amidine] hydrate, 2,2′-azobis {2- [1- (2-droxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2,2′-azobis [2- (2-imidazoline- 2-yl) propane], 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, 2,2′-azobis {2 Methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2′-azobis [2-methyl-N- (2-hydroxyethyl) propionamide], 4,4 And '-azobis (4-cyanovaleric acid).

The chain transfer agent having a hydrophilic functional group is not particularly limited, and examples thereof include mercaptopropanediol having a hydroxyl group, thioglycerol having a carboxyl group, mercaptosuccinic acid, mercaptoacetic acid, aminoethanethiol having an amino group, and the like. Examples thereof include thiophosphoric acid having an acid group.

The polymerization initiator having a hydrophilic functional group is not particularly limited. For example, P-menthane hydroperoxide (manufactured by NOF Corporation: Permenta H), diisopropylbenzene hydroperoxide (Japan) Oil Co., Ltd .: Parkmill P), 1,2,3,3-tetramethylbutyl hydroperoxide (1,2,3,3-tetramethyl hydroxide) (manufactured by NOF Corporation: Perocta H), cumene hydroperoxide (Manufactured by NOF Corporation: Parkmill H-80), t-butyl hydroperoxide (manufactured by NOF Corporation: PERBUCHI H-69), cyclohexanone peroxide (manufactured by NOF Corporation: Perhexa H), 1,1,3,3-tetramethylbutyl hydroperoxide, t-butyl hydroperoxide, t-amyl hydroper Examples thereof include oxide and disuccinic acid peroxide (parroyl SA). Various azo initiators having a nitrogen functional group may be used.

The presence of a hydrophilic functional group at the molecular end of the (meth) acrylic resin can be confirmed by, for example, ¹³ C-NMR measurement of a polymer extracted from the (meth) acrylic resin. That is, when a bond between carbon and sulfur is confirmed, it can be determined that a hydrophilic functional group is introduced at the molecular end.

Although the weight average molecular weight by polystyrene conversion of the said (meth) acrylic resin is not specifically limited, A preferable minimum is 5000 and a preferable upper limit is 500,000. When the weight average molecular weight is less than 5,000, the obtained conductive fine particle dispersed paste does not have a sufficient viscosity, and as a result, the ability to form a pattern on the substrate after the screen is removed is lowered. On the other hand, when the weight average molecular weight exceeds 500,000, the adhesive strength of the obtained conductive fine particle-dispersed paste becomes too high, so that the yarn may be generated and the screen printability may be deteriorated. The upper limit with the said preferable weight average molecular weight is 100,000, and a more preferable upper limit is 50000. The weight average molecular weight in terms of polystyrene can be obtained by performing GPC measurement using, for example, a column LF-804 (manufactured by SHOKO) as a column.
The weight average molecular weight in the present specification is, for example, a polystyrene equivalent value based on the results obtained by gel permeation chromatography (GPC) analysis at room temperature using LF-804 manufactured by SHOKO as a column. Means what is required in

The polyvinyl acetal resin is not particularly limited as long as it has excellent compatibility with the organic solvent described later, but is obtained by acetalizing a polyvinyl alcohol resin having a saponification degree of 80 mol% or more, The degree of polymerization is preferably 1000 to 4000 and the degree of acetalization is preferably 60 to 80 mol%.

The saponification degree of the polyvinyl alcohol is preferably 80 mol% or more. If the degree of saponification is less than 80 mol%, the acetalization reaction becomes difficult due to poor solubility of polyvinyl alcohol in water, and if the amount of hydroxyl groups is small, the acetalization reaction itself may be difficult. is there.

The degree of polymerization of the polyvinyl alcohol is preferably 1000 to 4000.
When the polymerization degree is less than 1000, the strength of the coating film may be insufficient. If the degree of polymerization exceeds 4000, the solubility in water may decrease, or the viscosity of the aqueous solution may become too high, making acetalization difficult. In addition, the solution viscosity becomes too high and the coatability is lowered.
In addition, the polymerization degree of the said polyvinyl acetal uses the polymerization degree of polyvinyl alcohol which is a raw material at the time of synthesize | combining. Moreover, when mixing 2 or more types of polyvinyl alcohol, the average of these polymerization degrees is used.

The polyvinyl alcohol can be obtained by saponifying a vinyl ester polymer.
Examples of the vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate and the like, and vinyl acetate is preferable from the economical viewpoint.
The polyvinyl alcohol preferably contains an α-olefin in the main chain.
Since the hydrogen bonding force of the polyvinyl acetal resin is weakened by the α-olefin, the viscosity stability over time can be improved, and the screen printability can be improved.

Examples of the α-olefin include methylene, ethylene, propylene, isopropylene, butylene, isobutylene, pentylene, hexylene, cyclohexylene, cyclohexylethylene, and cyclohexylpropylene, and ethylene is particularly preferable.
As content of the said alpha olefin, it is desirable that it is 1-20 mol%.
When the content of the α-olefin is less than 1 mol%, the properties of the obtained polyvinyl acetal resin are no different from those of an unmodified polyvinyl acetal resin, and when it exceeds 20 mol%, the solubility of polyvinyl alcohol in water As a result, the acetalization reaction may become difficult, and the resulting polyvinyl acetal resin may become too hydrophobic to reduce its solubility in organic solvents.

As said polyvinyl alcohol, you may use what copolymerized the other ethylenically unsaturated monomer in the range which does not impair the effect of this invention.
Examples of such ethylenically unsaturated monomers include acrylic acid, methacrylic acid, (anhydrous) phthalic acid, (anhydrous) maleic acid, (anhydrous) itaconic acid, acrylonitrile methacrylonitrile, acrylamide, methacrylamide, and trimethyl. -(3-acrylamido-3-dimethylpropyl) -ammonium chloride, acrylamido-2-methylpropanesulfonic acid and its sodium salt, ethyl vinyl ether, butyl vinyl ether, N-vinyl pyrrolidone, vinyl chloride, vinyl bromide, vinyl fluoride , Vinylidene chloride, vinylidene fluoride, tetrafluoroethylene, sodium vinyl sulfonate, sodium allyl sulfonate and the like.
Also use terminal-modified polyvinyl alcohol obtained by copolymerizing vinyl ester monomer such as vinyl acetate with ethylene in the presence of thiol compounds such as thiol acetic acid and mercaptopropionic acid, and saponifying it. Can do.

Although the aldehyde used for the said reaction is not specifically limited, For example, formaldehyde (including paraformaldehyde), acetaldehyde (including paraacetaldehyde), propionaldehyde, butyraldehyde, amylaldehyde, hexylaldehyde, heptylaldehyde, 2-ethylhexylaldehyde, Examples include cyclohexyl aldehyde, furfural, glyoxal, glutaraldehyde, benzaldehyde, 2-methylbenzaldehyde, 3-methylbenzaldehyde, 4-methylbenzaldehyde, p-hydroxyaldehyde, m-hydroxyaldehyde, phenylacetaldehyde, phenylpropionaldehyde and the like.
These aldehydes may be used alone or in combination of two or more, and acetaldehyde and / or butyraldehyde is preferably used.

The polyvinyl acetal resin is obtained by dissolving polyvinyl alcohol with warm water, adding an aldehyde so as to have a predetermined degree of acetalization in the presence of an acid catalyst, reacting, and then washing, neutralizing, and drying. be able to.
The acid catalyst is not particularly limited, and any organic acid or inorganic acid can be used. Examples thereof include acetic acid, paratoluenesulfonic acid, nitric acid, sulfuric acid, and hydrochloric acid.
Moreover, as an alkali used for neutralization, sodium hydroxide, potassium hydroxide, ammonia, sodium acetate, sodium carbonate, sodium hydrogencarbonate, potassium carbonate etc. are mentioned, for example.

The degree of acetalization of the polyvinyl acetal resin used in the present invention is preferably 60 to 80 mol%. When the degree of acetalization is less than 60 mol%, the hydrogen bondability of the polyvinyl acetal resin becomes too strong, and sufficient screen printability may not be obtained. On the other hand, the polyvinyl acetal resin having a degree of acetalization exceeding 80 mol% is generally difficult to produce industrially. (The maximum acetalization degree is preferably 81.6 mol% according to the theoretical calculation by PJ Flory. J. Am. Chem. Soc., 61, 1518 (1939)).

In the conductive fine particle-dispersed paste of the present invention, the content of at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin and polyvinyl acetal resin is not particularly limited. A preferred lower limit is 0.2% by weight and a preferred upper limit is 10% by weight. When the content of at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin and polyvinyl acetal resin is less than 0.2% by weight, the viscosity of the obtained conductive fine particle dispersed paste is too low. The ability to form a pattern on the substrate after the screen is removed may decrease. On the other hand, when the content exceeds 10% by weight, the adhesive strength of the obtained conductive fine particle dispersed paste is too high, and stringing may occur when the screen is removed. A more preferred lower limit is 0.5% by weight, and a more preferred upper limit is 5% by weight.

The conductive fine particle-dispersed paste of the present invention contains an organic compound that is solid at room temperature, has two or more hydroxyl groups, and (carbon number / hydroxyl number) is less than 5.

The organic compound is solid at room temperature. When the organic compound is solid at room temperature, the conductive fine particle-dispersed paste obtained can have a high viscosity, and the storage stability of the paste can be improved.
The organic compound is solid at room temperature, but a paste-like conductive fine particle dispersion paste can be obtained by dissolving in an organic solvent described later.
The organic compound is different from the organic solvent described later.

The organic compound has at least two hydroxyl groups. By having two or more hydroxyl groups, the interaction with the conductive fine particles becomes strong and the dispersion stability can be improved, so that the storage stability of the paste can be improved.
If the number of hydroxyl groups is one or less, the interaction with the conductive fine particles cannot be sufficiently exerted, and the effect of improving the storage stability of the conductive fine particle-dispersed paste may not be expected.
The organic compound preferably contains two or more hydroxyl groups.

The organic compound has (carbon number / hydroxyl number) of less than 5. When the above (the number of carbon atoms / the number of hydroxyl groups) is 5 or more, the contribution of the hydroxyl groups becomes small, so that interaction with the conductive fine particles may not be expected. Preferably it is less than 4.5. The preferred lower limit of the above (number of carbon atoms / number of hydroxyl groups) is 1.

The organic compound is not particularly limited as long as it is solid at room temperature, has two or more hydroxyl groups, and (carbon number / hydroxyl number) is less than 5, but from an aliphatic chain having 5 to 20 carbon atoms. A polyhydric alcohol-based organic compound such as diol or triol is preferable.
The polyhydric alcohol organic compound is not particularly limited. For example, 2,2-dimethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1 , 3-propanediol, trimethylolethane, trimethylolpropane and the like.
Among these, 2,2-dimethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, and trimethylolpropane, which have a high ratio of hydroxyl group to carbon number, can be preferably used.

The content of the organic compound in the conductive fine particle-dispersed paste of the present invention is not particularly limited, but a preferred lower limit is 1% by weight and a preferred upper limit is 25% by weight. When the content of the organic compound is less than 1% by weight, the effect of dispersing and stabilizing the conductive fine particles is low, and the effect of improving the storage stability of the resulting conductive fine particle dispersed paste may be insufficient. Moreover, when content of the said organic compound exceeds 25 weight%, an organic compound may stop melt | dissolving in a paste.

The conductive fine particle dispersed paste of the present invention contains an organic solvent.
The organic solvent is not particularly limited, for example, ethylene glycol ethyl ether, ethylene glycol monobutyl ether, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoisobutyl ether, trimethylpentanediol monoisobutyrate, Butyl carbitol, butyl carbitol acetate, texanol, isophorone, butyl lactate, dioctyl phthalate, dioctyl adipate, benzyl alcohol, phenylpropylene glycol, cresol, terpineol, terpine acetate, dihydroterpineol, dihydroterpineol acetate, 1,3-butanediol 1,5-pentanediol, -Methoxy-3-methyl-1-butanol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2-ethyl- Examples include 1,3-hexanediol and glycerin. Especially, it is preferable to contain the terpene solvent which has at least one hydroxyl group in a molecule | numerator, or the polyhydric alcohol solvent whose (carbon number / hydroxyl number) is less than five. By using such an organic solvent in combination with at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin and polyvinyl acetal resin, and the organic compound, molecules of the (meth) acrylic resin The storage stability of the resulting conductive fine particle dispersed paste can be further improved by the interaction between the hydrophilic functional group present at the terminal and the hydroxyl group in the organic solvent and organic compound. As such an organic solvent, terpineol, 3-methyl-1,3-butanediol, 2,4-diethyl-1,5-pentanediol, and 2-ethyl-1,3-hexanediol are preferably used.
In addition, these organic solvents may be used independently and may use 2 or more types together.

The conductive fine particle dispersed paste of the present invention contains conductive fine particles.
Examples of the conductive fine particles include metal powders such as aluminum powder, gold powder, silver powder, copper powder, nickel powder, platinum powder and palladium powder, and metal oxide powders such as alumina, zinc oxide and ferrite. Of these, aluminum powder is preferred.

The aluminum powder preferably has an average particle diameter of 0.5 to 20 μm and a substantially spherical shape. When the average particle size is less than 0.5 μm, the specific surface area of the aluminum powder is increased, and when it exceeds 20 μm, the dispersion stability may be lowered when a conductive fine particle-dispersed paste is obtained. When applied to a silicon wafer, the contact between the silicon wafer and the aluminum powder decreases, and a uniform aluminum-silicon alloy layer may not be obtained after firing. More preferably, it is 1-10 micrometers. The substantially spherical shape includes particles having a shape close to a sphere in addition to a true sphere shape.

Moreover, as said aluminum powder, it is preferable to use what combined the aluminum particle with an average particle diameter of 0.5-5 micrometers and the aluminum particle with an average particle diameter of 5-20 micrometers in arbitrary ratios. Thereby, the packing of the aluminum particles in the film obtained after sintering becomes denser.

The content of the conductive fine particles in the conductive fine particle-dispersed paste of the present invention is not particularly limited, but a preferable lower limit with respect to the entire conductive fine particle-dispersed paste is 60% by weight, and a preferable upper limit is 85% by weight. If the content of the conductive fine particles is less than 60% by weight, the resulting conductive fine particle-dispersed paste is too low in viscosity to be suitable for screen printing, or the sintered aluminum film Resistance may increase, leading to a decrease in energy conversion efficiency of the solar cell. On the other hand, when the content of the conductive fine particles exceeds 85% by weight, the applicability of the conductive fine particle-dispersed paste during screen printing may deteriorate. A more preferred lower limit is 65% by weight, and a more preferred upper limit is 80% by weight.

The conductive fine particle-dispersed paste of the present invention preferably further contains glass frit. For example, when the glass frit is applied to a silicon substrate, it is considered that the glass frit functions to further strengthen the bond between the aluminum electrode layer and the silicon substrate. The glass frit content is preferably 5% by weight or less. If the content of the glass frit exceeds 5% by weight, the glass may be segregated. The glass frit is most preferably non-lead glass that does not adversely affect the environment, but lead-containing glass may also be used. The average particle size of the glass frit particles is not particularly limited, but is preferably 20 μm or less.

The conductive fine particle-dispersed paste of the present invention may contain other conventionally known additives as necessary within the range not impairing the effects of the present invention.

The method for producing the conductive fine particle-dispersed paste of the present invention is not particularly limited. For example, the (meth) acrylic resin, the organic solvent, the conductive fine particles, and an additive to be added as necessary are conventionally used, such as a three roll. It can manufacture by the method of mixing for a predetermined time using a well-known stirrer.
The method of mixing is not particularly limited, and examples thereof include a method of mixing using a homodisper, a universal mixer, a Banbury mixer, a kneader and the like.

Since the conductive fine particle-dispersed paste of the present invention has an appropriate viscosity characteristic, it is excellent in screen printability and can be suitably used for the production of a back electrode of a solar cell. In addition, since it can be a paste having excellent storage stability without increasing the resin content in the paste, it can be stored stably over a long period of time, and has excellent electrical characteristics. can do.

According to the present invention, the conductive fine particle dispersed paste can be stably stored for a long period of time, and the storage stability can be improved. Moreover, since the electrical resistance value can be reduced, a conductive fine particle dispersed paste excellent in electrical characteristics can be obtained.

It is sectional drawing which shows the structure of a common solar cell typically.

Examples of the present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

(Polymerization example 1)
In a 2 L separable flask equipped with a stirrer, cooler, thermometer, hot water bath, and nitrogen gas inlet, 100 parts by weight of isobutyl methacrylate (IBMA), 0.4 parts by weight of mercaptopropanediol as a chain transfer agent, organic solvent As a result, 100 parts by weight of terpineol was added to obtain a monomer mixture.

The obtained monomer mixture was bubbled with nitrogen gas for 20 minutes to remove dissolved oxygen, and then the temperature inside the separable flask system was replaced with nitrogen gas and heated until the hot water bath boiled with stirring. As a polymerization initiator, 0.1 part by weight of an organic oxide polymerization catalyst (Perroyl 355, manufactured by NOF Corporation) was added, a polymerization initiator was added several times during the polymerization, and the total was 100 parts by weight of the monomer. 1.5 parts by weight of a polymerization initiator was added.
Seven hours after the start of polymerization, the polymerization was terminated by cooling to room temperature. As a result, a terpineol solution of a polymer of isobutyl methacrylate having a hydroxyl group at the terminal (Poly (IBMA)) was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, and the weight average molecular weight in terms of polystyrene was 40,000.

(Polymerization example 2)
As monomers, 50 parts by weight of methyl methacrylate (MMA), 50 parts by weight of cyclohexyl methacrylate (CHMA), 0.8 parts by weight of mercaptopropanediol as a chain transfer agent, and 2,4-diethyl-1,5-pentanediol as an organic solvent 100 A 2,4-diethyl-1,5-pentanediol solution of a (meth) acrylic resin (Poly (MMA / CHMA)) having a hydroxyl group at the molecular end in the same manner as in Polymerization Example 1 except that parts by weight were used. Got. When the obtained polymer was analyzed by gel permeation chromatography using Column LF-804 (manufactured by Showa Denko KK) as a column, the weight average molecular weight in terms of polystyrene was 20,000.

(Polymerization example 3)
In a 2 L separate flask equipped with a stirrer, cooler, thermometer, oil bath and nitrogen gas inlet, 90 parts by weight of isobutyl methacrylate (IBMA), 10 parts by weight of hydroxyethyl methacrylate (HEMA), 100 parts by weight of terpineol as an organic solvent Were mixed to obtain a monomer mixture.

The obtained monomer mixture was bubbled with nitrogen gas for 20 minutes to remove dissolved oxygen, and then the temperature inside the separable flask system was replaced with nitrogen gas and heated up until the oil bath reached 130 ° C. while stirring. . As a polymerization initiator, a solution in which 2,2'-azobis [2-methyl-N- (2-hydroxyethyl) propionamide] was dispersed with terpineol was added. Further, a terpineol solution containing a polymerization initiator was added several times during the polymerization.

Seven hours after the start of polymerization, the reaction solution was cooled to room temperature to complete the polymerization. This obtained the terpineol solution of the (meth) acrylic resin (Poly (IBMA / HEMA)) which has an amide group in a molecule terminal.
When the obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, the weight average molecular weight in terms of polystyrene was 35,000.

(Polymerization example 4)
As in Polymerization Example 3, except that 80 parts by weight of methyl methacrylate (MMA), 20 parts by weight of hydroxyethyl methacrylate (HEMA) were used as the monomer, and 100 parts by weight of 2-ethyl-1,3-hexanediol was used as the organic solvent. Thus, a 2-ethyl-1,3-hexanediol solution of a (meth) acrylic resin (Poly (MMA / HEMA)) having an amide group at the molecular end was obtained. When the obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, the weight average molecular weight in terms of polystyrene was 30,000.

(Polymerization Example 5)
As monomers, methyl methacrylate (MMA) 40 parts by weight, isobutyl methacrylate (IBMA) 50 parts by weight, hydroxyethyl methacrylate (HEMA) 10 parts by weight, chain transfer agent 0.5 parts by weight mercaptopropanediol, and organic solvent 100 parts by weight terpineol A terpineol solution of a (meth) acrylic resin (Poly (MMA / IBMA / HEMA)) having a hydroxyl group at the molecular end was obtained in the same manner as in Polymerization Example 1 except that the part was used. When the obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, the weight average molecular weight in terms of polystyrene was 35,000.

(Polymerization Example 6)
As in Polymerization Example 3, except that 40 parts by weight of methyl methacrylate (MMA), 40 parts by weight of cyclohexyl methacrylate (CHMA), 20 parts by weight of hydroxyethyl methacrylate (HEMA) and 100 parts by weight of terpineol as the organic solvent were used as monomers. Thus, a terpineol solution of a (meth) acrylic resin (Poly (MMA / CHMA / HEMA)) having an amide group at the molecular end was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, and the weight average molecular weight in terms of polystyrene was 40,000.

(Polymerization Example 7)
As monomers, 50 parts by weight of methyl methacrylate (MMA), 50 parts by weight of cyclohexyl methacrylate (CHMA), 0.8 part by weight of mercaptopropanediol as a chain transfer agent, and 100 parts by weight of butyl carbitol acetate (BCA) as an organic solvent were used. Except for this, a BCA solution of (meth) acrylic resin (Poly (MMA / CHMA)) having a hydroxyl group at the molecular end was obtained in the same manner as in Polymerization Example 1. When the obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, the weight average molecular weight in terms of polystyrene was 30,000.

(Polymerization Example 8)
Except for using 40 parts by weight of methyl methacrylate (MMA), 40 parts by weight of cyclohexyl methacrylate (CHMA), 20 parts by weight of hydroxyethyl methacrylate (HEMA) as a monomer, and 100 parts by weight of butyl carbitol acetate (BCA) as an organic solvent, In the same manner as in Polymerization Example 3, a BCA solution of a (meth) acrylic resin (Poly (MMA / CHMA / HEMA)) having an amide group at the molecular end was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, and the weight average molecular weight in terms of polystyrene was 40,000.

(Polymerization Example 9)
Polymerization examples, except that 60 parts by weight of isobutyl methacrylate (IBMA), 40 parts by weight of cyclohexyl methacrylate (CHMA), 0.6 parts by weight of mercaptopropanediol as a chain transfer agent, and 100 parts by weight of terpineol as an organic solvent were used as monomers. In the same manner as in Example 1, a terpineol solution of a (meth) acrylic resin (Poly (IBMA / CHMA)) having a hydroxyl group at the molecular end was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, and the weight average molecular weight in terms of polystyrene was 40,000.

(Polymerization Example 10)
A (meth) acryl having an amide group at the molecular end in the same manner as in Polymerization Example 3 except that 100 parts by weight of cyclohexyl methacrylate (CHMA) is used as the monomer and 100 parts by weight of butyl carbitol acetate (BCA) is used as the organic solvent. A BCA solution of resin (Poly (CHMA)) was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by Showa Denko KK) as a column, and the weight average molecular weight in terms of polystyrene was 40,000.

Example 1
Ethylcellulose STD4 was dissolved in terpineol. To this terpineol solution, 2,2-dimethyl-1,3-propanediol was added as an organic compound so as to have the composition ratio shown in Table 1, and dispersed with a high-speed agitator to obtain a vehicle composition.

Aluminum fine particles (average particle size 5 μm) and glass frit (average particle size 1 μm) as conductive fine particles were added to the obtained vehicle composition so as to have the composition ratio shown in Table 1, and a high-speed stirring device was added. After using and kneading sufficiently, treatment was performed with a three roll mill while paying attention not to flatten the aluminum fine particles, and a conductive fine particle dispersed paste was prepared.

(Example 2)
A 3-methyl-1,3-butanediol solution of ethylcellulose STD45 was used in place of the terpineol solution of ethylcellulose STD4, and trimethylolpropane was used in place of 2,2-dimethyl-1,3-propanediol as the organic compound. A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that the composition ratio was changed to 1.

(Example 3)
Example 1 except that the terpineol solution of (meth) acrylic resin (Poly (IBMA)) obtained in Polymerization Example 1 was used instead of the terpineol solution of ethyl cellulose STD4, and the composition ratio was changed to that shown in Table 1. Thus, a conductive fine particle dispersed paste was prepared.

Example 4
Instead of the terpineol solution of ethyl cellulose STD4, a 2,4-diethyl-1,5-pentanediol solution of the (meth) acrylic resin (Poly (MMA / CHMA)) obtained in Polymerization Example 2 was used. A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that trimethylolpropane was used instead of 2-dimethyl-1,3-propanediol and the composition ratio was changed to those shown in Table 1.

(Example 5)
Example 1 except that the terpineol solution of (meth) acrylic resin (Poly (IBMA / HEMA)) obtained in Polymerization Example 3 was used instead of the terpineol solution of ethyl cellulose STD4, and the composition ratio was changed to those shown in Table 1. In the same manner, a conductive fine particle dispersed paste was prepared.

(Example 6)
Instead of the terpineol solution of ethyl cellulose STD4, a 2-ethyl-1,3-hexanediol solution of (meth) acrylic resin (Poly (MMA / HEMA)) obtained in Polymerization Example 4 was used, and the composition ratio shown in Table 1 was obtained. A conductive fine particle dispersion paste was prepared in the same manner as in Example 1 except that the change was made.

(Example 7)
Instead of the terpineol solution of ethyl cellulose STD4, a terpineol solution of (meth) acrylic resin (Poly (MMA / IBMA / HEMA)) obtained in Polymerization Example 5 was used, and 2,2-dimethyl-1,3-propane was used as the organic compound. A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that trimethylolpropane was used in place of the diol and the composition ratio was changed to that shown in Table 1.

(Example 8)
Instead of the terpineol solution of ethyl cellulose STD4, a terpineol solution of (meth) acrylic resin (Poly (MMA / CHMA / HEMA)) obtained in Polymerization Example 6 was used, and 2,2-dimethyl-1,3-propane was used as the organic compound. Conductivity was the same as in Example 1 except that trimethylolpropane was used instead of diol, terpineol and 3-methyl-1,3-butanediol were used as the additional solvent, and the composition ratios were changed to those shown in Table 1. A fine particle dispersed paste was prepared.

Example 9
Using a solution of ethyl cellulose STD4 dissolved in terpineol and a terpineol solution of (meth) acrylic resin (Poly (IBMA / HEMA)) obtained in Polymerization Example 3, the composition ratio was changed to the composition ratio shown in Table 1. A conductive fine particle dispersed paste was prepared in the same manner as in Example 1 except that.

(Example 10)
Using a solution in which ethylcellulose STD7 was dissolved in terpineol and a terpineol solution of (meth) acrylic resin (Poly (MMA / CHMA / HEMA)) obtained in Polymerization Example 6, terpineol and 3-methyl-1, A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that 3-butanediol was used and the composition ratio was changed to the composition ratio shown in Table 1.

(Example 11)
(Synthesis of polyvinyl acetal resin)
193 g of polyvinyl alcohol having a polymerization degree of 1700 and a saponification degree of 98 mol% was added to 2900 g of pure water and stirred at a temperature of 90 ° C. for about 2 hours for dissolution.
This solution is cooled to 40 ° C., 20 g of hydrochloric acid having a concentration of 35% by weight and 145 g of n-butyraldehyde are added thereto, the temperature of the solution is lowered to 15 ° C., and this temperature is maintained to carry out an acetalization reaction. The product was precipitated.
Thereafter, the liquid temperature was kept at 40 ° C. for 3 hours to complete the reaction, and neutralized, washed with water and dried by a conventional method to obtain a white powder of polyvinyl acetal resin.
When the obtained polyvinyl acetal resin was dissolved in DMSO-d ₆ (dimethyl sulfoxide) and the degree of acetalization was measured using ¹³ C-NMR (nuclear magnetic resonance spectrum), the degree of acetalization was 78 mol%. there were.

2 parts by weight of the obtained polyvinyl butyral resin is added to 11 parts by weight of terpineol and dissolved by stirring. Further, 1 part by weight of dibutyl phthalate as a plasticizer and trimethylolpropane as an organic compound have a composition ratio shown in Table 1. In addition, it was dissolved by stirring with a high-speed stirring device. Aluminum fine particles (average particle size 5 μm) and glass frit (average particle size 1 μm) as conductive fine particles were added to the obtained vehicle composition so as to have the composition ratio shown in Table 1, and a high-speed stirring device was added. After using and kneading sufficiently, treatment was performed with a three roll mill while paying attention not to flatten the aluminum fine particles, and a conductive fine particle dispersed paste was prepared.

(Comparative Example 1)
Except that ethylcellulose STD10 was used instead of ethylcellulose STD4, 2,2-dimethyl-1,3-propanediol as an organic compound was not used, and the composition ratio was changed to the composition ratio shown in Table 1, the same as in Example 1 was used. A conductive fine particle dispersion paste was prepared.

(Comparative Example 2)
Instead of the terpineol solution of ethyl cellulose STD4, the terpineol solution of (meth) acrylic resin (Poly (IBMA)) obtained in Polymerization Example 1 was used, and 2,2-dimethyl-1,3-propanediol as an organic compound was used. First, a conductive fine particle dispersion paste was prepared in the same manner as in Example 1 except that the composition ratio was changed to the composition ratio shown in Table 1.

(Comparative Example 3)
2,2-dimethyl-1,3-propanediol as an organic compound using a BCA solution of (meth) acrylic resin (Poly (MMA / CHMA)) obtained in Polymerization Example 7 instead of a terpineol solution of ethyl cellulose STD4 A conductive fine particle dispersed paste was prepared in the same manner as in Example 1 except that the composition ratio was changed to the composition ratio shown in Table 1.

(Comparative Example 4)
Instead of the terpineol solution of ethyl cellulose STD4, a BCA solution of (meth) acrylic resin (Poly (MMA / CHMA / HEMA)) obtained in Polymerization Example 8 was used, and 2,2-dimethyl-1,3-as an organic compound was used. A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that propanediol was not used and the composition ratio was changed to that shown in Table 1.

(Comparative Example 5)
A conductive fine particle-dispersed paste was prepared in the same manner as in Example 11 except that trimethylolpropane as an organic compound was not used and the composition ratio was changed to that shown in Table 1.

(Comparative Example 6)
Instead of the terpineol solution of ethyl cellulose STD4, the terpineol solution of (meth) acrylic resin (Poly (IBMA / CHMA)) obtained in Polymerization Example 9 was used, and 2,2-dimethyl-1,3-propanediol was used as the organic compound. Instead, a conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that 2-methyl-1,3-propanediol was used and the composition ratio was changed to that shown in Table 1.

(Comparative Example 7)
Instead of 2,2-dimethyl-1,3-propanediol as an organic compound, the BCA solution of (meth) acrylic resin (Poly (CHMA)) obtained in Polymerization Example 10 was used instead of the terpineol solution of ethyl cellulose STD4. A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that myristyl alcohol was used and the composition ratio was changed to that shown in Table 1.

(Evaluation)
The conductive fine particle dispersed pastes obtained in Examples and Comparative Examples were evaluated by the following methods. The results are shown in Table 2.

(1) Screen printing property The obtained conductive fine particle dispersed paste was applied to a screen printing machine ("MT-320TV" manufactured by Microtech), screen plate making ("ST200" emulsion 12 [mu] manufactured by Tokyo Process Service Co., Ltd.), screen frame: 320 mm x 320 mm) and a printed glass substrate (soda glass: 150 mm × 150 mm, thickness: 1.5 mm), printing was performed in an environment of a temperature of 23 ° C. and a humidity of 50%.
○ If the screen printing plate does not leave a trace on the glass substrate and a clean printing surface is obtained, the glass substrate sticks to the screen plate making and a clean printing surface cannot be obtained, or conductive fine particle dispersed paste on the screen plate making Was left in large quantities and could not be printed.

(2) Sinterability A substrate obtained by printing a conductive fine particle dispersed paste on a silicon wafer (thickness: 300 μm) by screen printing under the same printing conditions as in “(1) Screen printability” in an oven at 730 to 750 ° C. Firing was carried out by holding for 2 minutes. The case where the silicon wafer or the conductive fine particle dispersed paste was distorted, warped or cracked, or the strength of the sintered film was weak and peeled off was evaluated as x.

(3) Electrical resistance value The left and right terminals of a digital tester (made by Custom, CDM-6000) were applied to the sintered film formed on the silicon wafer after firing, and the electrical resistance value was measured.

(4) Storage stability The obtained conductive fine particle dispersed paste was stored under constant temperature and humidity of 23 ° C. and humidity of 50%. One month after confirmation, the paste without separation or sedimentation was marked with ◯, and the separation and sedimentation was marked with ×.

ADVANTAGE OF THE INVENTION According to this invention, since it is excellent in storage stability and can make an electrical resistance value low, the electroconductive fine particle dispersion | distribution paste which can be used suitably for the back surface electrode formation of a solar cell can be provided.

1 p-type silicon semiconductor substrate 2 n-type impurity layer 3 antireflection film 4 grid electrode 5 back electrode layer 6 Al—Si alloy layer 7 p ⁺ layer

Claims (8)

A conductive fine particle dispersion paste used to form a back electrode of a solar cell,
Containing at least one selected from the group consisting of ethyl cellulose, (meth) acrylic resin, and polyvinyl acetal resin, an organic solvent, an organic compound that is solid at room temperature, and conductive fine particles,
The conductive fine particle-dispersed paste, wherein the room temperature solid organic compound is an organic compound that is solid at room temperature, has two or more hydroxyl groups, and has (carbon number / hydroxyl number) less than 5. Organic compounds that are solid at room temperature are 2,2-dimethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylol. The conductive fine particle-dispersed paste according to claim 1, wherein the paste is at least one selected from the group consisting of propane. The organic solvent is a terpene solvent having at least one hydroxyl group in a molecule, or a polyhydric alcohol solvent having (carbon number / hydroxyl number) of less than 5, 3 or 3. Conductive fine particle dispersion paste. 4. The conductive fine particle-dispersed paste according to claim 1, wherein the conductive fine particles are aluminum powder having an average particle diameter of 0.5 to 20 μm and a substantially spherical shape. 5. The conductive fine particle-dispersed paste according to claim 1, wherein the content of the aluminum powder is 60 to 85% by weight. Furthermore, glass frit is contained, The electroconductive fine particle dispersion paste of Claim 1, 2, 3, 4 or 5 characterized by the above-mentioned. A solar battery cell produced using the conductive fine particle dispersed paste according to claim 1, 2, 3, 4, 5 or 6. A solar cell panel produced by using the conductive fine particle dispersed paste according to claim 1, 2, 3, 4, 5 or 6.

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